Whistler Waves As a Signature of Converging Magnetic Holes in Space Plasmas

TL;DR

This study uses high-resolution space observations to identify how electron interactions generate whistler waves within magnetic holes, revealing their role in plasma structure evolution.

Contribution

It uncovers the local generation mechanism of whistler waves in magnetic holes through combined Landau and cyclotron resonances, supported by observational and numerical evidence.

Findings

Whistler waves are generated by electron beam interactions in magnetic holes.

Counter-streaming electron beams form due to betatron and Fermi effects.

Observations agree with numerical predictions from ALPS.

Abstract

Magnetic holes are plasma structures that trap a large number of particles in a magnetic field that is weaker than the field in its surroundings. The unprecedented high time-resolution observations by NASA's Magnetospheric Multi-Scale (MMS) mission enable us to study the particle dynamics in magnetic holes in the Earth's magnetosheath in great detail. We reveal the local generation mechanism of whistler waves by a combination of Landau-resonant and cyclotron-resonant wave-particle interactions of electrons in response to the large-scale evolution of a magnetic hole. As the magnetic hole converges, a pair of counter-streaming electron beams form near the hole's center as a consequence of the combined action of betatron and Fermi effects. The beams trigger the generation of slightly-oblique whistler waves. Our conceptual prediction is supported by a remarkable agreement between our observations and numerical predictions from the Arbitrary Linear Plasma Solver (ALPS). Our study shows that wave-particle interactions are fundamental to the evolution of magnetic holes in space and astrophysical plasmas.